Note: Descriptions are shown in the official language in which they were submitted.
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PROCESSING OF METAL OR ALLOY OBJECTS
FIELD OF THE INVENTION
The present invention relates to the processing of objects, in particular
objects made of metals or alloys.
BACKGROUND
Metals and metal alloys are used in many market sectors, including the
aerospace, medical and sports and leisure sectors.
The manufacture of metal or alloy objects may be performed by
machining processes or a combination of forging and machining processes.
Objects may also be made using casting and/or powder metallurgy routes, for
example using a metal injection moulding process.
However, such manufactured objects, particularly those made by powder
metallurgy processes, may comprise micro-pores and other imperfections at or
proximate to the surface of the object. The presence of such imperfections
tends to adversely affect the fatigue performance of an object, especially in
high-cycle fatigue situations. For example, the imperfections may act as crack
initiators.
Hot isostatic pressing tends not to remove such imperfections if they are
connected to the surface.
SUMMARY OF THE INVENTION
In a first aspect, the present invention provides a method of processing
an object, the object being made of a metal or an alloy, the object having a
plurality of open cavities, the method comprising performing a sealing process
on the object to seal the openings of the open cavities, thereby forming a
plurality of closed cavities, and reducing the sizes of the closed cavities by
performing a consolidation process on the object having the closed cavities.
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The step of reducing the sizes of the closed cavities may be performed at
least until the closed cavities are no longer present in the object.
The step of performing a consolidation process may comprise performing
a hot isostatic pressing process.
The object may be an object that has been produced using a process
selected from a group of processes consisting of: net shape manufacturing
processes, near net shape manufacturing processes, powder metallurgy
processes, spray forming processes, metal injection moulding, direct metal
deposition, selective laser melting, additive layer manufacturing, casting,
rolling,
and forging.
The object may be an object that has been produced using a metal
injection moulding to form the object.
The object may be a brown stage object that has been sintered.
The step of performing a sealing process may comprise plastically
deforming the surface of the object.
Plastically deforming the surface of the object may comprise shot
peening the surface of the object.
The step of performing a sealing process may further comprise sintering
the object after the surface of the object has been plastically deformed.
The step of performing a sealing process may comprise coating the
surface of the object with a layer of material thereby providing a coated
object,
wherein the material is a metal or alloy that is different to the metal or
alloy from
which the object is made.
The step of performing a sealing process may further comprise heating
the coated object such that atoms from the layer of material diffuse into the
object, and such that atoms from the object diffuse into the layer of
material.
The step of heating the coated object may comprise melting a portion of
the coated object, the portion being at or proximate to the surface of the
coated
object.
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The layer of material and the object may form a eutectic composition at
or proximate to the interface between the layer of material and the object.
The step of heating the coated object may comprise heating the coated
object to a temperature, the temperature being above a eutectic temperature of
the eutectic composition, and the temperature being below a melting point of
the metal or alloy from which the object is made.
The material may comprise copper.
The metal or alloy from which the object is made may be selected from a
group of metals or alloys consisting of: titanium alloys, steel, and aluminium
alloys.
In a further aspect, the present invention provides a method of producing
an object, the method comprising providing an initial object, the initial
object
being made of a metal or an alloy, the initial object having a plurality of
open
cavities, and processing the initial object using a method according to any of
the
above aspects, thereby providing the produced object.
In a further aspect, the present invention provides an object that has
been produced or processed using a method according to any of the above
aspects.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic illustration (not to scale) of an object;
Figure 2 is a process flow chart showing certain steps of a process of
producing the object;
Figure 3 is a schematic illustration (not to scale) of a cross section of a
portion of a sintered part;
Figure 4 is a schematic illustration (not to scale) of the cross section of
the portion of the sintered part after having been shot peened;
Figure 5 is a schematic illustration (not to scale) of the cross section of
the portion of the shot peened part after having been re-sintered;
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Figure 6 is a schematic illustration (not to scale) of the cross section of
the portion of the re-sintered part after having a hot isostatic pressing
process
performed on it;
Figure 7 is a process flow chart showing certain steps of a further
process of producing the object;
Figure 8 is a schematic illustration (not to scale) of a cross section of a
portion of a sintered part after having been coated with a copper layer;
Figure 9 is a schematic illustration (not to scale) of the cross section of
the portion of the copper coated part when heated; and
Figure 10 is a schematic illustration (not to scale) of the cross section of
the portion of the heated part after the copper layer has diffused into it.
DETAILED DESCRIPTION
Figure 1 is a schematic illustration (not to scale) of an object 2. The
object 2 is made of a titanium alloy. The object 2 may be any appropriate
object
e.g. a component part of a machine or machinery. The object has a surface 4. A
first embodiment of a process of producing the object 2 will now be described.
Figure 2 is a process flow chart showing certain steps of a first
embodiment of a process of producing the object 2.
At step s2, a metal injection moulding process is performed to produce a
so-called "green part".
In this embodiment, a conventional metal injection moulding process is
performed. A relatively finely-powdered alloy is mixed with binder material to
produce a so called "feedstock". This feedstock is shaped using an injection
mould process to produce the green part.
In this embodiment, the alloy is titanium with 6% aluminium and 4%
vanadium (also known as Ti-6A1-4V, or 6-4, 6/4, ASTM 8348 Grade 5).
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At step s4, after the green part is cooled and de-moulded, a portion of
the binder material is removed from the green part to produce a so-called
"brown part".
In this embodiment, a conventional process for removing binder material
from the green part is used, e.g. by using a solvent, a thermal evaporation,
and/or a catalytic process, etc.
In this embodiment, the brown part produced by the metal injection
moulding and binder removal processes has a solid density of approximately
60%. In other words, the brown part is relatively porous.
Also, the brown part has substantially uniform porosity throughout the
part. The surface and internal structure of the brown part have substantially
equal porosity.
At step s6, a sintering process is performed on the brown part. A
conventional sintering process is used.
In this embodiment, the brown part is sintered at a temperature in the
range 1000 C to 1300 C. Preferably, the brown part is sintered at a
temperature
in the range 1250 C to 1300 C This sintering process tends to agglomerate the
metal particles in the brown part, thereby increasing the solid density of the
part.
The component formed by sintering the brown part has a solid density
within the range 92% to 100%. In other words, the sintered brown part is
relatively solid. The terminology "solid" is used herein to refer to a
material that
has a density by volume (i.e. solid density) of between 92% and 100%.
The brown part after it has been sintered will hereinafter be referred to as
the "sintered part".
Figure 3 is a schematic illustration (not to scale) of a cross section of a
portion of the sintered part 6. The portion shown in Figure 3 is proximate to
the
surface of the sintered part 6 (which is the surface of the produced object 2
and
so is indicated in Figure 3 by the reference numeral 4)
The surface 4 of the sintered part 6 is relatively uneven, i.e. rough.
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Proximate to its surface 4, the sintered part 6 comprises a plurality of
closed cavities 8 (i.e. closed pores or voids in the material body). These
closed
cavities 8 are hollow spaces or pits in the body of the sintered part 6.
Furthermore, the closed cavities 8 are not open to the atmosphere, i.e. they
are
not connected to the surface 4. In other words, gas cannot flow from outside
the
sintered part 6 into the closed cavities 8 and vice versa.
The sintered part 6 further comprises a plurality of open cavities 10 (i.e.
open pores or voids in the material body). These open cavities 10 are cavities
or hollows that are open to the atmosphere, i.e. cavities or hollows that are
connected to the surface 4 such that gas can flow from outside the sintered
part
6 into the those open cavities.
The sintered part 6 may, for example, have an average surface
roughness of approximately 10pm with a periodicity of approximately 10-
20pm. Open cavities 10 may, for example, be up to 60pm deep. In other
embodiments, the open cavities 10 may, for example, extend into the sintered
part 6 from its surface 4 to a depth of up to 200pm.
In conventional methods, after the sintered part 6 has been formed, a hot
isostatic pressing (HIP) process is typically performed on the sintered part 6
to
reduce the porosity, and increase the density, of the part. Were a HIP process
performed on the sintered part 6 (as is performed conventionally), the
sintered
part 6 would be subjected to elevated temperature and elevated isostatic gas
pressure, e.g. by subjecting the sintered part 6 to a heated, pressurised gas
such as argon. Thus, there would be relatively high pressure on the surface 4
of
the sintered part 6, whilst there would be relatively low pressure in the
closed
cavities 8 (due to their not being open to the surface 4). The application of
heat
and the creation of a pressure differential between the atmosphere and the
closed cavities 8 would tend to cause the closed cavities 8 to shrink, or
vanish
completely. This may be due to a combination of plastic deformation, creep,
and
diffusion bonding caused by the elevated temperature and pressure. However,
a conventional HIP process performed on the sintered part 6 would tend not to
shrink, or remove, the open cavities 10 from the sintered part 6. The heated
pressurised gas applied to the sintered part 6 during the HIP process may flow
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into the open cavities 10. Thus, there would tend to be no pressure
differential
between the atmosphere and the open cavities 10, and the open cavities 10
would therefore not be closed by the HIP process.
This deficiency of conventional methods of producing objects/parts may
be overcome by performing steps s8 to s12 on the sintered part 6, as opposed
to just performing a HIP process.
At step s8, the sintered part 6 (produced by performing steps s2 to s6) is
shot peened.
In this embodiment, a conventional shot peening process is used. This
process comprises impacting a surface 4 of the sintered part 6 with shot (e.g.
substantially round particles made of metal, glass, or ceramic) with
sufficient
force such that the sintered part 6 is plastically deformed at its surface 4.
In this embodiment, any appropriate shot medium may be used, e.g.
S330 (cast steel with an average diameter of 0.8mm). Also, any appropriate
shot peening pressure may be used, e.g. 0.5bar, 0.75bar, 1.25bar, 2bar and
4bar. Also, any appropriate Almen intensities may be used, e.g. 0.15mmA,
0.20mmA, 0.30mmA, 0.38mmA and 0.52mmA.
Figure 4 is a schematic illustration (not to scale) of the cross section of a
portion of the sintered part 6 after having been shot peened. This part will
hereinafter be referred to as the "shot peened part" and is indicated in
Figure 4
by the reference numeral 12. The portion of the part shown in Figure 4 is the
same portion as shown in Figure 3.
The surface 4 of the show peened part 12 is relatively smooth (compared
to the surface 4 prior to shot peening).
Furthermore, the process of shot peening tends to plastically deform the
sintered part 6 at its surface 4 such that the openings of the open cavities
10
are either closed such that gas cannot flow from outside the sintered part 6
into
an open cavity 10 and vice versa (i.e. such that, in effect, an open cavity 10
becomes a closed cavity 8), or are closed such that the opening of an open
cavity 10 to the surface 4 is very small but that gas may still flow from
outside
the sintered part 6 into an open cavity 10 and vice versa.
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In this embodiment, the plastic deformation of the surface of the sintered
part 6 is performed by shot peening. However, in other embodiments a different
plastic deformation process is used, for example, a process of burnishing e.g.
using a roller.
At step s10, the shot peened part 12 is re-sintered.
A conventional sintering process, such as that used at step s6, may be
used. For example, the sintering of the shot peened part 12 may comprise
sintering at a temperature in the range 1000 C to 1300 C. and preferably at a
temperature in the range 1250 C to 1300 C. The sintering process is performed
for a time period at a temperature for diffusion bonding the compacted open
cavities lOnear the surface, for example in the range 750-1400 C.
Figure 5 is a schematic illustration (not to scale) of the cross section of a
portion of the shot peened part 12 after having been re-sintered. This part
will
hereinafter be referred to as the "re-sintered part" and is indicated in
Figure 5 by
the reference numeral 14. The portion of the part shown in Figure 5 is the
same
portion as shown in Figures 3 and 4.
The sintering of the shot peened part 12 tends to agglomerate the metal
particles of the shot peened part. In particular, the sintering process tends
to
diffusion bond the openings of the open cavities 10 (that were either closed
or
almost closed by the shot peening process) such that, in effect, the open
cavities 10become closed cavities 8 (as shown in Figure 5). In other words,
the
openings of the open cavities 10 are fully sealed by sintering the part 12,
i.e. the
re-sintering of the shot peened part 12 tends to close the open cavities 10
such
that gas cannot flow from outside the shot peened part 12 into an open cavity
10 and vice versa. In other words, the open cavities 10 are made impermeable
to fluids.
At step s12, a hot isostatic pressing (HIP) process is performed on the
re-sintered part 14.
A conventional HIP process is used to reduce the porosity, and increase
the density, of the re-sintered part 14. In this embodiment, the re-sintered
part
14 is subjected to elevated temperature and elevated isostatic gas pressure by
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subjecting the re-sintered part 14 to heated and pressurised argon. A HIP
cycle
having a duration of approximately 2 hours, a temperature of 920 C, and a
pressure of 102MPa may be used. Figure 6 is a schematic illustration (not to
scale) of the cross section of a portion of the re-sintered part 14 after
having a
HIP process performed on it. The hot isostatic pressing of the re-sintered
part
14 produces the object 2. The portion of the part shown in Figure 6 is the
same
portion as shown in Figures 3 to 5
The HIP process produces a relatively high pressure at the surface 4 of
the re-sintered part 14, whilst the pressures in the closed cavities 8
(including
the open cavities 10 that have been formed into closed cavities 8 as described
above) are relatively low. This is due to the closed cavities 8 not being open
to
the surface 4, i.e. being gas-tight. As a result of plastic deformation,
creep,
and/or diffusion bonding caused by the elevated temperature and pressure, the
closed cavities 8 in the re-sintered part shrink or vanish completely.
The hot isostatic pressing of the re-sintered part 14 produces the object =
2. Thus, a process of producing the object 2 is provided.
In the above described first embodiment, the object 2 is produced using
a shot peening and re-sintering treatment. A second, alternative, embodiment
of
a process of producing the object 2 in which a different treatment will now be
described.
Figure 7 is a process flow chart showing certain steps of a second
embodiment of a process of producing the object 2.
At step s14, a metal injection moulding process is performed to produce
a green part. This is done as described above with reference to step s2 of
Figure 2.
At step s16, a portion of the binder material is removed from the green
part to produce a brown part. This is done as described above with reference
to
step s4 of Figure 2.
At step s18, a sintering process is performed on the brown part to
produce a sintered part 6. This is done as described above with reference to
step s6 of Figure 2.
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The sintered part 6 at step s18 is as described above with reference to
Figure 3.
At step s20, the surface 4 of the sintered part 6 is coated, or plated, with
a layer of copper.
The coating of the surface of the sintered part 6 may be performed using
any appropriate coating or plating process, for example electro-plating.
Figure 8 is a schematic illustration (not to scale) of the cross section of a
portion of the sintered part 6 after having been coated with a copper layer
16.
This part will hereinafter be referred to as the "coated part" and is
indicated in
Figure 8 by the reference numeral 18. The portion of the part shown in Figure
8
is the same portion as shown in Figure 3.
In this embodiment, the copper layer 16 covers the entire surface 4 of the
sintered part 6.
At step s22, the coated part 18 is heated.
At the interface between the titanium alloy part and the copper layer 16,
i.e. at the surface 4, titanium atoms tend to diffuse into the copper layer 16
and
copper atoms tend to diffuse into the titanium alloy. At some point at or near
the
interface between the titanium alloy and copper layer a eutectic composition
is
formed, i.e. a layer of a eutectic composition tends to form. This eutectic
composition of titanium and copper has a lower melting temperature than the
titanium alloy from which the sintered part 6 is formed. This eutectic
composition also has a lower melting temperature than the copper layer.
The heating of the coated part 18 at step s22 is performed such that the
coated part 18 is heated to above the melting point of the eutectic
composition.
In other words, the coated part 18 is heated to above the eutectic temperature
of the titanium/copper composition.
Thus, at step s24, the eutectic composition of titanium and copper
formed at the surface 4 of the sintered part 6 melts.
Figure 9 is a schematic illustration (not to scale) of the cross section of a
portion of the coated part 18 heated to above the eutectic temperature of the
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titanium/copper eutectic composition. A molten, i.e. liquid, layer 20 is
formed at
the interface between the titanium alloy material and the copper layer 16.
This
part will hereinafter be referred to as the "heated part" and is indicated in
Figure
9 by the reference numeral 22. The portion of the part shown in Figure 9 is
the
same portion as shown in Figures 3 and 8.
As the heating of heated part 22 is continued, more and more titanium
and copper tends to dissolve into the liquid layer 20 and the thickness of the
liquid layer 20 increases until the entire solid copper layer 16 has been
dissolved into the liquid layer 20.
to Also, as heating of the heated part 22 is continued, copper atoms tend
to
diffuse into the titanium alloy material away from the surface 4. Also, more
titanium atoms tend to diffuse into the liquid layer 20. Thus, the proportion
of
titanium in the liquid layer 20 tends to increase. This change in the
composition
of the liquid layer 20 tends to increase its melting temperature. Thus, the
liquid
layer 20 solidifies.
Thus, at step s26, after a certain amount of time being heated, the
material at the surface of the heated part 22 solidifies. In other words, the
copper layer 16 has diffused into the titanium alloy material (and vice versa)
to
such a degree that the melting point of the titanium/copper composition is
greater than the eutectic temperature, and greater than the temperature to
which the heated part 22 is heated.
Figure 10 is a schematic illustration (not to scale) of the cross section of
a portion of the heated part 22 after the copper layer 16 has diffused into
it, and
the surface of the molten layer 20 has solidified. The portion of the part
shown
in Figure 10 is the same portion as shown in Figures 3, 8 and 9.
The dissolution of the outer surface of the titanium part into the liquid
layer 20 together with the subsequent re-solidification of that layer tends to
close the openings of the open cavities 10 such that, in effect, the open
cavities
10 become closed cavities 8 (as shown in Figure 10). In other words, after the
copper layer 16 has diffused into the titanium alloy material, and the surface
of
the heated part 22 has solidified, the openings of the open cavities 10 are
fully
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sealed i.e. such that gas cannot flow from outside the heated part 22 into an
open cavity 10 and vice versa. In other words, the open cavities 10 are made
impermeable to fluids.
The heating of the heated part 22 may be performed until the copper is
substantially uniformly diffused throughout the heated part 22.
The surface 4 of the heated part 22 is relatively smooth (compared to the
surface 4 of the sintered part 6).
At step s28, a hot isostatic pressing (HIP) process is performed on the
re-sintered part 14. This is done as described above with reference to step
s12
of Figure 2.
The HIP process tends to cause the closed cavities 8 in the part shrink or
vanish completely as described in more detail above with reference to step s12
of Figure 2.
The hot isostatic pressing of the heated part 22 produces the object 2.
The object 2 produced using the method of the second embodiment comprises
an amount of copper. Thus, a further process of producing the object 2 is
provided.
An advantage provided by the above described methods is that pores,
pits, or other (e.g. minute) openings, orifices, or interstices in the surface
of the
object tend to be removed. In other words, defects and/or discontinuities at
or
proximate to the surface of the object may, in effect, be repaired.
Conventional
processes of performing a hot isostatic pressing process on a sintered part
tend
not to remove such open cavities. These open cavities may act as crack
initiators. Thus, removal of these open cavities from the object tends to
result in
improved fatigue performance, especially in high-cycle fatigue situations. The
improved surface finish and microstructure of the object tend to improve its
fatigue performance.
The above described methods also tend to remove (or shrink) the closed
cavities (or other voids or hollows that are closed to the surface) in the
body of
the object. This also tends to improve the microstructure of the object, which
tends to lead to improved fatigue performance.
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A further advantage provided by the above described methods is that the
surface finish of the object tends to be improved. The object tends to be
shinier
than those that are produced using conventional techniques. This increased
reflectivity is important in certain applications. For example, if the object
is for
decorative purposes, the improved aesthetic appearance of the object tends to
be important.
A further advantage provided by the above described processes is that
an object may be produced using a powder metallurgy manufacturing
technique. This tends to provide that a near-net-shape component is produced
with very little wastage. Furthermore, it tends to be relatively easy to make
relatively complex shapes that may be prohibitively expensive to machine.
The above described processes are advantageously applicable to
objects of any size. This is because a treatment process (i.e. a process of
shot
peening, re-sintering, and hot isostatic pressing, or a process of coating,
heating, and hot isostatic pressing) is performed after the formation of the
object
(i.e. after the alloy powder has been sintered).
A further advantage provided by the above described processes is that
any of the treatment processes may be performed on a large number of objects
simultaneously. Thus, a cost of performing any or all of these operations (per
component) may be significantly reduced.
In the second embodiment, the thickness of the copper layer may be
small in comparison to the size of the object. Thus, the amount of copper used
in the process of Figure 7 is relatively small compared to the amount of
titanium
alloy. Advantageously, the amount of copper is so small that diffusion of that
amount of copper into the titanium alloy (as described above with reference to
steps s24 and steps s26 of Figure 7) tends not to adversely affect the
mechanical properties of the titanium alloy object to any significant degree.
Advantageously, the above described process tends to seal the surface
of the object, and so make that object more amenable to a HIP process. The
above described process may advantageously be applied objects that have
open porosity throughout the body of the object. In such applications, an
initial
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sinter (i.e. the sintering of the brown part performed at step s6 or s18 of
the
above described embodiments) may be performed at a lower temperature
and/or for a shorter time.
It should be noted that certain of the process steps depicted in the
flowcharts of Figures 2 and 7 and described above may be omitted or such
process steps may be performed in differing order to that presented above and
shown in those Figures. Furthermore, although all the process steps have, for
convenience and ease of understanding, been depicted as discrete temporally-
sequential steps, nevertheless some of the process steps may in fact be
performed simultaneously or at least overlapping to some extent temporally.
In the above embodiments, the object is formed using a process
comprising a metal injection moulding process. However, in other embodiments
the object is formed using a different process. For example, an object may be
manufactured using one or a combination of the following processes: a
machining process, a forging process, a casting process, a powder metallurgy
process. Also for example, the object may be formed using a different net-
shape or near-net shape manufacturing process. The terminology "near-net
shape manufacturing process" is used herein to refer to processes in which the
initial production of the item is (substantially) the same as, or very close
(i.e.
within allowed tolerances) to, the final (net) shape. This tends to reduce the
need for surface finishing of the object. For example, in other embodiments an
object may be produced using one or more of the following near-net shape
manufacturing processes: casting, permanent mould casting, powder
metallurgy, linear friction welding, metal injection moulding, rapid
prototyping,
spray forming, and superplastic forming. Such processes may comprise using
other powder metallurgy processes. Such processes may include, for example,
hot isostatic pressing (HIP), cold isostatic pressing (CIP), and 3D powder
melt
methods using scanning laser or electron beams. Such process may be used to
form a fully or partially consolidated metal or alloy object. Such processes
may
use feedstock produced by a conventional ingot route, or they may use solid
feedstock materials, such as a billet, plate, or bar made from lower cost,
higher
oxygen alloy powder via a powder metallurgy route. The metal/alloy powders
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used to produce the object may, for example, be blended elemental powders.
For example, an object that is made of Ti-6A1-4V can be produced from a
blended elemental powder made by blending powders of titanium, aluminium
and vanadium. Blended elemental powders tend to alloy and homogenise
during a sintering process. An object that is made of Ti-6A1-4V can also be
produced from a blended elemental powder made by blending titanium powder
with an Al-V master alloy powder.
In other embodiments, a treatment process (e.g. a process of shot
peening, re-sintering, and hot isostatic pressing, or a process of coating,
heating, and hot isostatic pressing) may be performed on any appropriate
object
e.g. an object with an undesirably irregular surface and/or internal defects
that
cannot be closed by hot isostatic pressing because they are connected to the
surface. The objects may be, for example, made of titanium alloys, steels or
aluminium alloys. The object may, for example, have a solid or partially solid
shape or form. The object may have been produced using any process, for
example near net shape processing, powder metallurgy, spray forming, metal
injection moulding, direct metal deposition, selective laser melting, additive
layer
manufacturing, casting, rolling, forging etc.
In the above embodiments, the object is formed from an alloy comprising
titanium with 6% aluminium and 4% vanadium (also known as Ti-6AI-4V, or 6-4,
6/4, ASTM B348 Grade 5). However, in other embodiments, the object is
formed from a different material. For example, in other embodiments, the
object
is formed from a pure (i.e. unalloyed) metal, or a different type of alloy to
that
used in the above embodiments.
In the above embodiments, the treatment processes (i.e. a process of
shot peening, re-sintering, and hot isostatic pressing, or a process of
coating,
heating, and hot isostatic pressing) are performed on a single object.
However,
in other embodiments, a treatment process, or part of a treatment process, may
be performed on any number of (different or the same) objects. This
advantageously tends to reduce the cost of the process per component.
In the above embodiments, the sintering (including re-sintering) of the
object is performed at the above specified temperatures, and for the above
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specified time-periods. However, in other embodiments sintering of an object
is
performed at a different appropriate temperature and/or for a different
appropriate time period.
In the above embodiments, the HIP process is performed at the above
specified temperatures and pressures, and for the above specified time-
periods.
However, in other embodiments a HIP process is performed at a different
appropriate temperature and/or pressure, and/or for a different appropriate
time
period.
In certain of the above embodiments, the surface of the sintered part is
coated, or plated, with a layer of copper. This is done to form a eutectic
composition at the surface of the part. However, in other embodiments, the
surface of the part is coated with a different substance so as to form a
different
eutectic composition at the surface of the part.
Also, in other embodiments, the surface of the part is coated with a
different substance that does not form a eutectic composition with titanium.
For
example, in another embodiment, the surface of the part is coated with a layer
of aluminium. The aluminium melts at a lower temperature than the titanium
alloy material. After having been coated with a layer of aluminium, the coated
part may be heated to a temperature that is above the melting point of
aluminium, but is below the melting point of the titanium alloy. Thus, a
liquid
layer of material is formed over the surface of the sintered part, i.e. the
surface
of the sintered part is ''wetted" by the molten aluminium. Titanium atoms tend
to
diffuse into the molten aluminium layer, and aluminium atoms tend to diffuse
into the titanium alloy body. After a certain amount of diffusion, the
openings of
the open cavities tend to be closed and the method may then proceed as
described above. In such embodiments, the sintered part may be produced
from a titanium alloy containing less than the desired proportion of
aluminium.
The diffusion of the aluminium layer into the part may be such that, after the
diffusion, the proportion of aluminium in the part is increased to the desired
level
(e.g. such that the part, after having an aluminium layer diffused into it,
has the
composition of Ti-6A1-4V). Furthermore, an allowable composition range for
aluminium in Ti-6A1-4V tends to be sufficiently large to allow or for an
object
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made of Ti-6AI-4V to absorb a significant amount of extra aluminium and it
still
meet the composition specification.
Also, in other embodiments, instead of allowing all of the material used to
coat/plate the sintered part (e.g. all of the copper, aluminium etc.) to
diffuse into
the sintered part, a portion of this coating material may be removed from the
surface of the part, e.g. by washing, acid pickling or evaporating it off.
In the above embodiments, the sealing process performed on the object
to seal the openings of the open cavities (i.e. the process of shot peening
and
sintering, or the process of coating and heating) is performed once before the
HIP process is performed on the object. However, in other embodiments, before
the HIP process is performed, one or both of the sealing processes may be
performed multiple times. For example, the sealing process of shot peening and
sintering may be performed more than once. In such an example, the sintering
process that follows a shot peening process, tends to soften the work hardened
surface formed during shot peening and tends to disperse any surface
contamination into the bulk of the object, making the surface of the object
more
amenable to another shot peening process. Furthermore, the second, and any
subsequent, shot peening processes may be performed at a lower intensity
than the first shot peening process. This tends to result in a better surface
appearance.
